There is a related art image forming apparatus which includes a process cartridge having a photosensitive drum rotating with holding a developer image thereon; an apparatus main body to or from which the process cartridge is attachable or detachable; and a driving force transmission mechanism transmitting a driving force from a driving source provided in the apparatus main body to the process cartridge. Specifically, the driving force transmission mechanism in the related art includes an input member rotatably attached in the process cartridge; a rotation driving shaft rotatably attached in the apparatus main body; and a driving force transmission member rotating together with the rotation driving shaft and being able to move forward/backward relative to the process cartridge in a parallel direction with a rotation axis of the rotation driving shaft.
Moreover, in the related art, a diameter of a tip end of the rotation driving shaft becomes smaller than that of a rear end of the rotation driving shaft. In this way, when the driving force transmission member moves forward, there occurs a gap between the driving force transmission member and the tip end of the rotation driving shaft. Accordingly, the driving force transmission member may swing with respect to the rear end thereof. Therefore, even when a central axis of the rotation driving shaft and a central axis of the input member become more or less deviated from each other, the rotation driving shaft and the input member are reliably engaged with each other, so that the driving force is transmitted to the process cartridge.